Press moulding method for forming an optical element

Abstract: In forming a plano-convex lens from a column-like lens blank by heating the blank to a temperature higher than the transition temperature thereof and by pressurizing said heated blank with a forming surface of a forming mold. The formed optical element has a center thickness of at least the initial center thickness of the unheated blank: the process for achieving this is one whereby the pressurising force for deformation has a component towards the center of an end surface of the heated blank.

Description: Background of the Invention The present invention relates to an optical element forming method and more particularly to a method of forming an economical optical element of high precision as to shape and surface and suitable for mass-production. To produce an optical element such as a lens, a prism or the like, there is recently known, instead of a method of grinding an optical element blank such as glass, plastic or the like, a method of previously preparing an optical element blank in a predetermined shape, feeding this blank in a forming mold, heating and then forming the blank under pressure (for example, Japanese Laid-Open Patent Publication No. 58-84134). As to such an optical element blank to be used for forming under pressure, the shape thereof is desired to be as simple as possible in view of workability and cost. As a typical example, there is proposed a forming method with the use of a column-like blank (for example, Japanese Laid-Open Patent Publication No. 3-50126). In a conventional forming method, to obtain an optical element having a predetermined center thickness Y and a predetermined optically effective surface diameter ⊘EFF, there has been used a column-like blank of which center thickness X is greater than Y and of which both end surfaces and lateral surface are mirror-finished. This disadvantageously increases the cost of the optical element. An optical element forming method comprising the features of the preamble of claim 1 is known from the JP-A-03295825. Summary of the Invention It is an object of the present invention to provide a method of economically producing a high-precision optical element in a stable manner. To achieve the object above-mentioned, the present invention provides a method as defined in claim 1. Further, to form an optical element provided outside of the optically effective surface thereof with a coarse surface, preferably there is selected, as the optical element blank, a cylindrical body having a coarse lateral surface and mirror-finished end surfaces each having a diameter ⊘D greater than the optically effective surface diameter ⊘EFF of the resulting optical element. According to the inventive method, the center thickness X of the blank can be determined with no restrictions exerted thereto by the center thickness Y of the resulting optical element. Brief Description of the Drawings Fig. 1 is a section view of a forming apparatus used in the method of the present invention; Fig. 2 is a perspective view of an optical element blank selected for use in the method of the invention; Fig. 3 is a perspective view of an optical element formed by the method of the invention; and Fig. 4 is a profile view illustrating the relationship between pressure and a period of time when conducting an operation of forming under pressure according to the method of the invention; Detailed Description of the Invention The following description will discuss embodiments of the present invention with reference to the attached drawings. The following description will discuss how to form a convexo-convex lens. In a forming apparatus in Fig. 1, a forming mold has an upper die 3, a lower die 4 and a trunk die 5. The upper die 3 and the lower die 4 are vertically slidably disposed in the trunk die 5 such that these dies 3, 4, 5 are coaxial. An optical element blank 1 is to be fed into a space formed by the upper die 3, the lower die 4 and the trunk die 5. The lower mold forming surface 4a is a concave surface. As shown in Fig. 2, an optical element blank 1 is a cylindrical body having a pair of opposite end surfaces 11 which are mirror-finished surfaces, and a lateral coarse surface 12. As shown in Fig. 1, the blank 1 is placed in the forming mold such that the end surfaces 11 are respectively opposite to an upper mold forming surface 3a which is a concave surface, and to a lower mold forming surface 4a which is also a concave surface. Thus, there are formed a closed space 3b between the blank 1 and the upper die 3, and a closed space 4b between the blank 1 and the lower die 4. Then, the heating sources incorporated in a pressurizing stage 6 and a forming stage 7 are turned on to heat the blank 1 to a predetermined temperature, and a pressurizing force is applied to the upper die 3 in a pressurizing pattern as shown in Fig. 2. Thus, the blank 1 is formed into an optical element (convexo-convex lens) 2 which has a predetermined thickness, mirror surfaces 21 and a coarse surface 22, as shown in Fig. 3. Here, as shown in Fig. 2, the center thickness of the optical element blank 1 is defined as X, and the diameter of each end surface or mirror surface as ⊘D. Further, as shown in Fig. 3, the center thickness of the formed optical element 2 is defined as Y, and the diameter of each mirror surface 21 as ⊘D'. It is noted that ⊘D' is substantially equal to ⊘D. In view of optical performance, the optically effective surface diameter ⊘EFF of the optical element 2 is required to be not greater than ⊘D'. That is, it is required that the coarse surface 22 is located outside of the optically effective surface of the optical element 2. To satisfy such conditions, the embodiment employs an optical element blank 1 which satisfies the requirements of X≦Y and ⊘D≧⊘EFF. When the volume of the optical element blank 1 is defined as V, the following equation is established: ⊘D = 2 (V/πX) 1/2 The following description will discuss a specific example of forming an optical element (convexo-convex lens) 2 having the center thickness of 2.9 mm and an optically effective surface diameter of 6.86 mm. The optical element blank 1 is a cylindrical body of optical glass VC-79 having a diameter of 7.1 mm and a thickness of 2.75 mm. The blank 1 was supplied onto the lower mold forming surface 4a, and the upper die 3 was then inserted along the trunk die 5 such that the upper mold forming surface 3a came in contact with the blank 1. At this time, the upper and lower peripheries of the blank 1 came in linear contact with the upper mold forming surface 3a and the lower mold forming surface 4a, respectively. Then, the heating sources were turned on to heat the blank 1 for 3 minutes at 590°C. At this point of time, the surface viscosity of the blank 1 was 10 8 poise. While lowering the temperature of the heating sources to 570°C, pressure was supplied to the pressurizing stage 6 and the upper die 3 started pushing the optical element blank 1. There was adopted a pressurizing method presenting a pressurizing pattern as shown in Fig. 4, in which the pressure was set to zero several times during one cycle. More specifically, there were repeated, five times, the application of a pressure of 285 kgf for five seconds and stopping the application of pressure for one second. During such repetition, gas in the closed spaces 3b, 4b defined by the end surfaces of the blank 1 and the forming surfaces 3a, 4a, was discharged. In the course of deformation under pressure, the contact parts of the blank 1 with the upper and lower dies 3, 4, i.e., the deformed parts of the blank 1, were gradually enlarged toward the center of the blank 1, and the transfer of the shapes of the upper and lower forming surfaces 3a, 4a proceeded. Further, the blank 1 was so deformed as to be outwardly extended by pressures to be exerted thereto in the vertical directions. Further, the blank 1 was made such that its surface and its inside were different in viscosity (the surface having viscosity at which the blank 1 was deformed, while the inside having viscosity at which the blank 1 was not deformed). Thus, in the course of the deformation of the surface of the blank 1, force to be decomposed toward the centers of the end surfaces of the blank 1, raised the center portions of the blank 1. At the final forming under pressure, with the pressurizing force maintained constant, pressure was supplied from the pressurizing stage 6 to the upper die 3 and the flange 3c of the upper die 3 was brought into close contact with the trunk die 5 previously adjusted such that the resulting optical element 2 had a desired thickness. About 40 seconds was taken before the upper die 3 came in close contact with the trunk die 5 after the pressure supply had started. At this point of time, the volume of the space defined by the dies was 315.23 mm 3 and the volume of the optical element (convexo-convex lens) 2 thus formed, was 108.88 mm 3 , which presented a volume rate of 34.5% with respect to the volume of the space defined by the dies. Thus, the space defined by the dies was not fully occupied by the optical element 2, and the circumferences of the optical element 2 did not come in contact with the inner surface of the trunk die 5. Then, when the pressurizing force was reduced to 55 kgf and the optical element 2 was cooled for two minutes to 480°C. the pressure supply was stopped to bring the pressurizing force to zero. When the temperature of the optical element 2 in the mold reached room temperature, the mold was opened and the optical element 2 was taken out. The optical element 2 thus formed had the center thickness of 2.9 mm which was greater by 150 µm than the center thickness of the blank 1 of 2.75 mm. The relationship between the center thickness X (2.75 mm) of the blank 1 and the center thickness Y (2.9 mm) of the optical element 2, satisfied X≦Y. Further, the relationship between the diameter ⊘D (7.1 mm) of each mirror end surface 11 of the blank 1 and the optically effective surface diameter ⊘EFF (6.86 mm) of the optical element 2, satisfied ⊘D≧⊘EFF. The forming method above-mentioned can be applied not only to a convexo-convex lens, but also a meniscus lens or the like.

Claims: An optical element forming method comprising: a step of heating an optical blank element placed in a forming mold having a concave forming surface to a mold temperature higher than the transition temperature of said blank; and a step of pressurising said heated blank with said forming surface of said forming mold so as to give, to said blank, force to be decomposed toward the center of an end surface of said blank such that said blank is deformed, under pressure, to an optical element characterized in that the forming method is conducted in such a way that the center thickness (Y) of the optical element is not less than the center thickness (X) of said blank. The method according to claim 1, further comprising a step of selecting a cylindrical body as the blank. The method according to claim 1, further comprising a step of selecting, as the blank, a cylindrical body having (i) a lateral surface which is a coarse surface and (ii) mirror-finished end surfaces each having a diameter greater than the optically effective surface diameter of the optical element, such that said coarse surface is located outside of said optically effective surface of said optical element.

Inventors: INOUE TAKASHI, TAKAGI KAZUAKI, YAMAGUSHI KENJI, INOUE, TAKASHI, TAKAGI, KAZUAKI, YAMAGUCHI, KENJI, YAMAGUSHI, KENJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: C03B11
IPC codes: C03B11